彻底看懂RocketMQ事务实现原理

public class CreateOrderService {

  @Autowired
  private OrderDao orderDao;
  @Autowired
  private ExecutorService executorService;

  private TransactionMQProducer producer;

  // 初始化transactionListener 和 producer
  @Init
  public void init() throws MQClientException {
    TransactionListener transactionListener = createTransactionListener();
    producer = new TransactionMQProducer("myGroup");
    producer.setExecutorService(executorService);
    producer.setTransactionListener(transactionListener);
    producer.start();
  }

  // 创建订单服务的请求入口
  @PUT
  @RequestMapping(...)
  public boolean createOrder(@RequestBody CreateOrderRequest request) {
    // 根据创建订单请求创建一条消息
    Message msg = createMessage(request);
    // 发送事务消息
    SendResult sendResult = producer.sendMessageInTransaction(msg, request);
    // 返回：事务是否成功
    return sendResult.getSendStatus() == SendStatus.SEND_OK;
  }

  private TransactionListener createTransactionListener() {
    return new TransactionListener() {
      @Override
      public LocalTransactionState executeLocalTransaction(Message msg, Object arg) {
        CreateOrderRequest request = (CreateOrderRequest ) arg;
        try {
          // 执行本地事务创建订单
          orderDao.createOrderInDB(request);
          // 如果没抛异常说明执行成功，提交事务消息
          return LocalTransactionState.COMMIT_MESSAGE;
        } catch (Throwable t) {
          // 失败则直接回滚事务消息
          return LocalTransactionState.ROLLBACK_MESSAGE;
        }
      }
      
      // 反查本地事务
      @Override
      public LocalTransactionState checkLocalTransaction(MessageExt msg) {
        // 从消息中获得订单ID
        String orderId = msg.getUserProperty("orderId");

        // 去db查询订单号是否存在，若存在则提交事务
        // 若不存在，可能是本地事务失败了，也可能是本地事务还在执行，所以返回UNKNOW
        return orderDao.isOrderIdExistsInDB(orderId)?
                LocalTransactionState.COMMIT_MESSAGE: LocalTransactionState.UNKNOW;
      }
    };
  }
}

public TransactionSendResult sendMessageInTransaction(final Message msg,
    final LocalTransactionExecuter localTransactionExecuter, final Object arg)
    throws MQClientException {
    TransactionListener transactionListener = getCheckListener();
    if (null == localTransactionExecuter && null == transactionListener) {
        throw new MQClientException("tranExecutor is null", null);
    }

    // ignore DelayTimeLevel parameter
    if (msg.getDelayTimeLevel() != 0) {
        MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_DELAY_TIME_LEVEL);
    }

    Validators.checkMessage(msg, this.defaultMQProducer);

    SendResult sendResult = null;
    // 给待发送消息添加属性，表明是一个事务消息（即半消息）
    MessageAccessor.putProperty(msg, MessageConst.PROPERTY_TRANSACTION_PREPARED, "true");
    MessageAccessor.putProperty(msg, MessageConst.PROPERTY_PRODUCER_GROUP, this.defaultMQProducer.getProducerGroup());
    // 像发送普通消息一样，把这条事务消息发往Broker
    try {
        sendResult = this.send(msg);
    } catch (Exception e) {
        throw new MQClientException("send message Exception", e);
    }

    LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
    Throwable localException = null;
    switch (sendResult.getSendStatus()) {
        // 事务消息发送成功
        case SEND_OK: {
            try {
                if (sendResult.getTransactionId() != null) {
                    msg.putUserProperty("__transactionId__", sendResult.getTransactionId());
                }
                String transactionId = msg.getProperty(MessageConst.PROPERTY_UNIQ_CLIENT_MESSAGE_ID_KEYIDX);
                if (null != transactionId && !"".equals(transactionId)) {
                    msg.setTransactionId(transactionId);
                }
                if (null != localTransactionExecuter) {
                    localTransactionState = localTransactionExecuter.executeLocalTransactionBranch(msg, arg);
                } else if (transactionListener != null) {
                    log.debug("Used new transaction API");
                    // 开始执行本地事务
                    localTransactionState = transactionListener.executeLocalTransaction(msg, arg);
                }
                if (null == localTransactionState) {
                    localTransactionState = LocalTransactionState.UNKNOW;
                }

                if (localTransactionState != LocalTransactionState.COMMIT_MESSAGE) {
                    log.info("executeLocalTransactionBranch return {}", localTransactionState);
                    log.info(msg.toString());
                }
            } catch (Throwable e) {
                log.info("executeLocalTransactionBranch exception", e);
                log.info(msg.toString());
                localException = e;
            }
        }
        break;
        case FLUSH_DISK_TIMEOUT:
        case FLUSH_SLAVE_TIMEOUT:
        case SLAVE_NOT_AVAILABLE:
            localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
            break;
        default:
            break;
    }

    // 事务过程的最后，给Broker发送提交或回滚事务的RPC请求。
    try {
        this.endTransaction(sendResult, localTransactionState, localException);
    } catch (Exception e) {
        log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", e);
    }

    TransactionSendResult transactionSendResult = new TransactionSendResult();
    transactionSendResult.setSendStatus(sendResult.getSendStatus());
    transactionSendResult.setMessageQueue(sendResult.getMessageQueue());
    transactionSendResult.setMsgId(sendResult.getMsgId());
    transactionSendResult.setQueueOffset(sendResult.getQueueOffset());
    transactionSendResult.setTransactionId(sendResult.getTransactionId());
    transactionSendResult.setLocalTransactionState(localTransactionState);
    return transactionSendResult;
}